Field of Invention
This invention relates generally to magnetic flowmeters, and more particularly to a flowmeter of this type which includes a ceramic flow tube or spool through which the fluid to be metered is conducted, the ceramic spool being metallized to define the measuring and shield electrodes of the flowmeter.
Magnetic flowmeters such as those disclosed in U.S. Pat. Nos. 3,695,104; 3,824,856; 3,783,687 and 3,965,738, are especially adapted to measure the volumetric flow rates of fluids which present difficult handling problems, such as corrosive acids, sewage and slurries. Because the instrument is free of flow obstructions, it does not tend to plug or foul.
In a magnetic flowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube or spool through which the fluid to be metered is conducted and to the transverse axis along which the measuring electrodes are located at diametrically-opposed positions with respect to the spool. The operating principles are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous value of the voltage established at the electrodes.
German patent publications DE-PS No. 24 54 469 and DE-PS No. 25 05 427 disclose magnetic flowmeters whose flow metering spool is fabricated of a non-magnetizable metal, the inner surface of the spool being covered by an electrical insulating liner. Embedded in this liner are the measuring and shield electrodes of the meter.
Embedment of the measuring and shield electrodes in the liner of the metering spool often results in hair line cracks and separation of the electrodes from the spool. This in turn gives rise to microphonic signals which adversely affect the measuring signal derived from the metering electrodes and degrade the accuracy of the flow rate reading.
Liners are often made of a fluorocarbon material, such as PTFE, this material being used because of its excellent chemical corrosion resistance properties. However, it is difficult to achieve adhesion between a fluorocarbon liner and the spool, and even a small negative pressure produced in the spool in the course of flow may cause separation therebetween.
In addition, microphonic noise often results from the cable connecting the metering electrodes to a pre-amplifier or an impedance converter having a high input impedance by reason of even small vibrations arising from the motion of the connecting cable relative to the spool.
Another disadvantage of prior art types of magnetic flowmeters is that the shield electrode which is intended to eliminate capacitive coupling between the metering electrodes and the spool does not cover the metering electrode to a sufficient extent to completely eliminate this undesirable coupling. An increase in the dimensions of the shield electrode in known forms of magnetic flowmeters is usually not feasible because the uncovered angle of the flow tube or spool is required for the electromagnetic system.